The present invention relates to an optical bidirectional module used for transmission and reception terminals for bidirectional optical fiber communication by means of a wavelength multiplex system.
In optical communication systems by means of optical transmission lines such as optical fiber lines, plural optical signals with different wavelengths are transmitted simultaneously through optical fiber by means of WDM (Wavelength Division Multiplex System), and bidirectional optical transmission modules are used in transmission and reception terminals for optical signals. A bidirectional optical transmission module of this kind is known by, for example, Patent Document 1 indicated below. This bidirectional optical transmission module contains light emitting element 100 and light receiving element 700 in a common package 120 and has holographic diffraction grating 600 on the upper or lower surface of cover glass 220 at an aperture of the package as shown in FIG. 5.
A light beam emitted from light emitting element 100 is transmitted through diffraction grating 600, and its zeroth-order diffracted light ray beam is focused on end facet 500 of optical fiber 400 by lens 300 in the transmission process. A light beam emitted from the end facet 500 of optical fiber 400 reaches diffraction grating 600 by way of lens 300 to be diffracted by diffraction grating 600 so that +first-order diffracted beam of the light may be focused on light detecting surface of light receiving element 700 and transmission signals from optical fiber 400 may be received in the reception process.
The diffraction grating 600 is structured as a two-level binary grating so that straight grating grooves 200 are formed on a flat plane of cover glass 220 (refractive index n) as shown in FIG. 6. However, when the two-level grating is employed for diffraction grating 600, it is difficult to maximize zeroth-order diffraction efficiency for another light with wavelength λ0, because each dimension of grating structure such as groove depth d, groove width W and pitch of grooves P are determined automatically based on the maximum condition of first-order diffracted light ray with wavelength λ1 which is given by λ1/{2(n−1)}.
[Patent Document 1] U.S. Pat. No. 5,555,334